The objectives of this research are the development and study of sets of recombinant inbred (RI) stains of mice, which were derived from the crosses of pairs of standard inbred (progenitor) strains. These RI strains will be used for the genetic analysis of numerous differences between the progenitors, including both simple qualitative differences such as electrophoretic protein variants and cell surface alloantigens, and more complex characters, such as resistance to infections. RI strains constitute a powerful genetic means to identify, map, and study the functions of individual genes. In this way the gormal genetics of the mouse is being extended. Restriction fragment length and minor histocompatibility loci polymorphisms will be used as additional sources of genetic markers.